Patent GB-1465091-A (GB)
Title: Transmission ratio control system
IPC: B60W30/18
Filed: 1974-03-20
Abstract: 1465091 Hydrostatic transmission control SUNDSTRAND CORP 20 March 1974 [19 April 1973] 12243/74 Heading F2W A hydrostatic transmission 14 for a tractor or truck driven by an internal combustion engine 10 is controlled by a valve 45 responsive to opposing fluid signals from an engine driven governor 72 and from either a valve 95 which supplies a signal responsive to fuel delivery pressure, accelerator pedal position or engine manifold depression or from an engine braking valve 135 which supplies a signal responsive to depression of a pedal 145. A valve spool 46 of the control valve 45 meters pressure liquid from an engine driven pump 64 through lines 55, 57 to a double acting servomotor 26 which adjusts the displacement of a swashplate pump 16 connected in closed circuit with a fixed displacement swashplate motor 18. The valve spool 46 is biased downwardly by engine speed responsive pressure in a chamber 70 and is normally biased upwardly by pressure responsive to fuel delivery pressure, accelerator pedal position or engine manifold depression in a chamber 94 which acts through a plunger 92 and spring 90 on the spool 46. An adjustable stop 97 limits the upward bias on the spool 46. The engine speed responsive pressure is applied to the chamber 70 from a valve 72 controlled by an engine driven centrifugal governor 77. The engine condition sensing valve 95, as shown, comprises a valve spool 110 biased right by a spring 125 and fuel delivery pressure supplied from a pump 32 to a valve chamber 127. The supply pressure 115 from the valve 95 acts through a duct 123 on the end of the valve spool 110 to move the latter left and provide a control pressure 115 proportional to fuel delivery pressure. The control pressure 115 is normally transmitted through a valve 120 to the chamber 94 of the ratio control valve 45. On depression of a brake pedal 145 pressure liquid is supplied to the top of the valve 120 to move its spool 155 down and thereby isolate the control pressure 115 from the chamber 94. With the valve spool 155 in its lower position the chamber 94 of the control valve is supplied with pressure liquid from a brake valve 135 at a pressure proportional to the depression of the pedal 145. Operation.-During normal operation the forces on the control valve spool 46 are balanced and maintain a ratio in the hydrostatic transmission which give minimum fuel consumption. An increase in engine speed or a decrease in fuel supply pressure causes downward movement of the valve spool 46 and a corresponding increase in pump displacement to restore the balance. Similarly a fall in engine speed or an increase in fuel supply pressure causes upward movement of the valve spool 46 and a decrease in pump displacement. If the accelerator pedal 40 is released the consequential reduction in fuel supply pressure causes the control valve spool 46 to move downwardly and increase pump displacement and thus minimize engine braking. On depression of the brake pedal 145 pressure liquid is supplied to the chamber 94 from the brake valve 135 to move the ratio control valve spool 46 upward and thus decrease the displacement of the pump 16 and consequentially maximize engine braking. Modifications.-In Fig. 3 (not shown) the valve spool of the engine condition valve 95 is biased rightwardly by a spring (175) adjustably loaded by a cam (165) actuated by a throttle pedal linkage. In Fig. 4 (not shown) the engine condition valve spool is loaded by a servo responsive to engine manifold depression.